linux-kernel - tty: n_gsm: race condition in gsmld

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Date: Tue, 16 Apr 2024 19:57:19 +0900
From: Yewon Choi <woni9911@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: "Dae R. Jeong" <threeearcat@...il.com>,
	syzbot+6e3e8f30f269f5028e5d@...kaller.appspotmail.com
Subject: tty: n_gsm: race condition in gsmld_ioctl

Hello,

memory leak in gsmld_ioctl was reported by syzbot, and this issue seems to be
raised from race condition involving gsm->dlci[addr].

https://syzkaller.appspot.com/bug?extid=6e3e8f30f269f5028e5d

When dlci object is checked and initialized in gsmld_ioctl(), it may be 
assigned multiple times because there is no lock which guards entering 
the critical section in gsm_dlci_alloc() from multiple threads.

For example, when multiple ioctl() are called concurrently, the following 
scenario is possible:

Thread 0                                Thread 1
ioctl(GSMIOC_SETCONF_DLCI)              ioctl(GSMIOC_GETCONF_DLCI)

gsmld_ioctl():                          gsmld_ioctl():
  dlci = gsm->dlci[addr];
                                          dlci = gsm->dlci[addr];
  if (!dlci) {
    gsm_dlci_alloc(gsm, addr):
      ...
      gsm->dlci[addr] = dlci;
                                          if (!dlci) {
                                            gsm_dlci_alloc(gsm, addr):
                                              ...
                                              // overwritten & memory leak
                                              gsm->dlci[addr] = dlci; 

We think either (1) gsm_dlci_alloc() should hold a lock(mutex) and do
internal check about whether gsm->dlci[addr] is NUll or not, OR 
(2) all callers of gsm_dlci_alloc() should hold gsm->mutex and check 
whether gsm->dlci[addr] is NUll or not (like gsmtty_install()).

Could you check this? If it makes sense, we will write a patch following
one of the suggestions.

Best Regards,
Yewon Choi